Microwave amplifier



Feb. E2, 1957 P. A. CLAVEER MICROWAVE AMPLIFIER Filed Dec. 15, 1950 INVENTOR PHUPPE A. CLAVIER ATTORNEY United States Patent MICROWAVE AMPLIFIER Philippe A. Clavier, Bayside, N. Y., assiguor to Sylvauia Electric Products Inc., a corporation of Massachusetts Application December 13, 1950, Serial No. 200,674

21 Claims. (Cl. SIS-3.5)

The present invention relates to amplifiers of the electron beam type suitable for amplifying signals in the microwave region of the radio spectrum, and may be broadly identified as travelling wave tubes.

Two types of travelling wave tubes have become widely known. A first type includes a helical conductor or waveguide extending parallel to an electron beam. The beam in this type interacts with the transmitted wave associated with the helix. A second type has two parallel electron beams of different beam velocities. These beams by their interaction, provide amplification of input signal modulation. The present invention introduces an additional type of travelling wave tube having certain basic diflerences from known travelling wave tubes.

1 have found that, if a broad and long electron beam of substantially uniform electron velocity throughout its cross-section is modulated as in other travelling wave tubes, gain will be realized where the cross-sectional space charge distribution of the beam is a non-linear variable, such as a variable diminishing from a maximum at a plane or axis of symmetry.

In an example, the electron beam can be projected along a cylindrical conductor, wave-guide or pipe and the space charge of the beam at the input end ofthe pipe can be distributed through its cross-section to have a maximum at the axis of the pipe, diminishing non-linearly and continuously along the radius of the beam and of the pipe. In such a tube, signal modulation will be amplified. For example, the space charge distribution may be assumed to be in the following form, as it leaves the electron gun:

( Po o+ 2 where p is the space-charge density, a and a are constants, and r is the radial coordinate.

With such a beam, the propagation constant 7 is:

2 Where and is the potential of the wave guide.

The gain, in such case, at the center-frequency is:

1/1 L Gdb=43 10 3 --2.e7

=w/ (Gaussian units) where I0 is the total current in amperes,

-L the active length of the pipe,

A the wave length at the center of the band, and the potential of the wave guide, in volts.

' modifications thereof. Figure 4 is an enlarged end view of the cathode in the embodiment of Figure 3.

In Figure 1 there is shown a travelling wave tube having an evacuated envelope 10 of glass or other suitable insulation. At one end of the envelope is an electron gun including a cathode 12 indirectly heated by filament 14, an accelerating grid 16 and focusing anodes 18. This gun produces an electron beam of substantial cross-section. The beam may actually be not appreciably larger in cross-section than in the helical type of travelling wave tube, but it is large relative to the bore diameter of the pipe 40 to be described, preferably large enough to fill that bore. A probe 20 and helix 22 serve together as the coupling device for impressing signal from a rectangular wave guide 24 on the electron beam. The electron beam passes within members 20 and 22 which are cylindrical and hollow. This form of coupling device adapts the tube to broad-band operation; but it can be replaced by apertures or grids as in klystrons for narrowband applications. A similar probe 26 at the opposite end of envelope 10 is joined to a similar coupling helix 28, elements 26 and 28 similarly constituting an output coupling devoce of hollow cylindrical form so as to afford a passage for the electron beam. The output coupling device is located within rectangular wave guide 30 resem- 7: In the foregoing expression,

to is the radian frequency,

v,, is the electron velocity,

R is the pipe radius, and

i is the electron charge-to-mass ratio.

bling wave-guide 24. (These wave-guides are sectioned in a plane that shows their narrow dimension, the long dimension of the wave-guide cross-section being perpendicular to the plane of the drawing.) Envelope 10 penetrates both of the wave-guides. Extensions 32 and 34 on waveguides 24 and 30 respectively co-act with probes 20 and 26 to constitute quarter-wave open-ended co-axial chokes which limit stray, endwise propagation of high frequency energy. The mode in guides 24 and 30 is TE01.

The electron beam that is produced by the cathode and by the accelerating and focusing electrodes is magnetically focused by coils 36 and 38 so that the beam projected along the axis of envelope 10 is maintained in beam form. Notably, the high intensity field of coil 36 extends to the cathode itself. Shield or conductive pipe or wave-guide 40 is connected to the input and output portions of larger diameter.

coupling devices. The beam travels through the coupling devices and the wave-guide, and is returned to the direct current supply by collector electrode 42. Electrodes 12, 14, 16, 18, 20 and 42 are provided with leads sealed through envelope 10 as is shown diagrammatically.

The beam that is formed by the electron gun including cathode 12 and electrodes 16 and 18 substantially fills the hollow cross-section of coupling elements 20, 22, 26, 28 and guide or shield 40. The aperture of the final focussing electrode 18 should be no smaller than the cross-section of the coupling elements and the pipe.

Shield 40 serves several purposes, but in principle is not an essential of novel travelling wave tubes. Shield 4i) prevents the accumulation of a space charge on the inside wall of envelope 10 which charge would tend to suppress the electron beam. The shield also affords :a beam path or drift space with defined and stabilized boundary conditions so that the high frequency energy associated with the beam is neither allowed toprop agate in stray paths nor is the beam subjected to stray influences of the surroundings. This shield is at times referred to as a waveguide, but it is normally not large enough to propagate the waves in the absence of the electron beam. In the presence of the beam, the shield guides the field propagation The surface of cathode 12 is of bullet-nose form with the small end much closer to flat grid 16 than the cathode The accelerating field of grid 16 at the cathode is much greater :at the axis than at points displaced radially and in consequence the beam current is greatest in the region of the axis and is reduced at the radial limits of the beam cross-section. The radialvelocity component of the emitted electrons can be disregarded because, when once accelerated by grid 16 so as to enter focussing electrode 18 as a beam, the radial position of each electron is maintained by focussing coil 36 whose strength should be sufficiently large to be considered infinite, as in other travelling wave tubes.

The following expression derived mathematically defines the optimum cross-sectional shape of cathode 12 when used with :a fiat grid. The distance d between any point on the cathode and the grid at a distance r from the axis, where R is the grid aperture, 1;) the positive grid voltage, 41 the cylinder voltage and a is a constant, all in Gaussian units, is:

However, in the final analysis, the desired space charge distribution of the beam is obtained from such a cathode shape, modified empirically. As pointed out above, this distribution along the beam radius should be neither uniform nor linearly decreasing but should diminish as a higher order function of the radial beam coordinate as in Equation 3. Considering the beam along its axial coordinate, the electron velocity need be only so high as to maintain the electron stream as a beam. of the desired space charge distribution. The gain of this. travelling wave tube depends upon the electron transit time within tube 40 and upon the beam velocity, so that a given gain can be realized with a shorter tube where a lower beam velocity can be established and controlled. Unlike the helical type of travelling wave tube, there is no restricting phase-velocity relation.

The type of beam involved is identifiable as a broad beam, in contrast to the theoretically desirable single line beam of helical travelling wave tubes; that is to say, there is a high electron density at the center of the beam, at its axisqand there is also a very real but continuously dimunshing electron density at points radially outward of the axis. By appropriate design of the cathode 12 in combination with the first electrode 16, any desired dis tribution of space charge density can be realized.

. a In Figure 2 an embodiment is shown for accomplishing the same bro-ad result by means of a different structure. The parts in Figure 2 which have the same reference numerals as in Figure l have the same functions and their description will not be repeated. However, grid electrode 16a of Wire mesh and cathode 12a are here modified so that the same distribution in electrostatic field of the grid at the cathode is realized as in Figure 1. Grid 16a here is of bullet-nose shape and cathode 12a is fiat. The space-charge density of the beam is thereby made a maximum at the tube axis and by appropriate design of the grid, the space charge distribution can be made to diminish non-linearly to the radial limit as established by shield 40 and the input and output probes.

In Figure 3 still another way of accomplishing the desired space charge distribution in the beam is realized with a flat cathode 12b and a flat grid 16b. In this form,

however, 'a center portion 12b is of high emissivity, as by a coating having a lower work function than that of the annular portion 12b". The cathode coating is applied as a thick mixture of rare earth carbonates later converted to oxides, and the two zones diffuse to some extent so that the center portion and the annular portion are not of sharply discontinuous work function, and there is no discontinuity in the cross-sectional space-charge distribution of the beam.

Portions 1212' and 12b" occupy different areas of the cathode. In reliance on higher operating temperatures that can be realized at portion 12b than at portion 1212", a higher emission characteristic can be realized :at the center than at the annular limit of the cathode surface. By providing a greater concentration of heating-wire at portion 12b than at portion 12b", and by providing a temperature gradient through greater heat dissipation at the periphery of the emissive surface via the heat-conducting and radiating cathode support, the desired nonlinearly emissive characteristic can be realized in a way alternative to or in addition to the differential workfunction coatings.

The variations of construction of the cathode described will be understood by those skilled in the art to provide electron-emission differential diminishing from a maximum at the center of the cathode to its outer extremities, for establishing the required space-charge distribution of the beam. The electron-emission and the transverse variations in electron emission in certain of these are spacecharge limited and others are temperature limited. Transversely variably emission-limited cathodes are embodied in all the forms shown. All three of the illustrated devices for achieving the desired space-charge distribution can be combined as, for example, by making both the grid and the cathode of bullet-nose form with the convex portions facing each other and with the convex end of the cathode coated with lower work function material than the outer cathode portions. In those arrangements where a flat grid is described, the wires represented in the drawing by dots can be omitted, the grid then simply constituting an electrode with one aperture; and there fore, the term grid as used herein is intended to identify that electrode in the gun nearest the cathode.

In the several embodiments above, it is apparent that the beam current-density is a function not only of the cathode construction but in addition it is dependent on the construction of the grid, and on the focussing voltage. The beam current density at ditferent parts of its cross-section is dependent on the shape of the grid in relation to the shape, the work-functions and the temperatureditferential of the cathode, and on the focussing electrode voltage to some extent. The special form of electron gun combines with the signal coupling devices andtlie structure affording the drift and the interaction space to produce amplification in a new way.

The beam cross-section and theshape of the tube 40 need not be circular butin general concept can assume any desired cross-sectional shape; and the symmetry of the beam described in the foregoing embodiments as being about the axis of the whole structure could, instead, be replaced by a beam having high space-charge density in a plane or other surface with the space-charge density diminishing with distance away from that surface, measured perpendicular to the beam path. The forms of travelling Wave tube described, which have long drift spaces compared to the cross-sectional dimension of the beam, presently require a magnetic focussing system and to this extent the circularly symmetric forms represent the preferred construction. A latitude of other variations will occur to those skilled in the art and, therefore,

the appended claims should be accorded that broad inter- I pretation that is consistent with the spirit and scope of the invention.

The form of input and output coupling probes disclosed herein is claimed in co-pending application Serial No. 54,676, filed October 15, 1948, by F. C. Breeden and L. C. Eisaman, and now Patent No. 2,660,690.

I claim:

1. An electron discharge device including a wave-guide, a cathode and accelerating-electrode gun structure for producing an electron beam adjacent to the input end of said waveguide, an electron collector at the output end of said Wave-guide, signal input and output coupling devices at the ends of said wave-guide, and magnetic focussing means extending from said cathode to said output coupling device for sustaining the cross-sectional form of the beam during its tnansit from the cathode to the collector through the wave-guide, said cathode and accelerating-electrode structure embodying means producing a beam of uniform velocity throughout its crosssection and of high electron density at its center and non-linearly decreasing electron density outward of the center.

2. An electron discharge device including an electron gun providing an electron beam of uniform electron velocity throughout its cross-section, means providing a drift space for said beam and maintaining the radial space-charge distribution of said beam in the form produced by the gun, and a collector at the end of said drift space remote from said gun, said gun including a cathode electrode and a grid electrode comprising means for producing an electron beam of maximum space charge den sity at the center and of reduced electron density at the outer extremities thereof.

3. An electron discharge device including a cathode and grid assembly producing an electron beam of uniform electron velocity throughout its cross-section and of high density in a predetermined path and non-linearly diminishing density at distances displaced from said path measured perpendiclular to the path, signal input and output coupling devices disposed in said path, and means affording a drift and interaction space, whereby signals coupled to the beam will be amplified by interaction of the high density and low density portions of the beam.

4. An electron discharge device in accordance with claim 3 in which said cathode has a high emission characteristic at its center and a progressive but non-linearly diminishing emission-limited characteristic at points displaced from its center.

5. An electron discharge device in accordance with claim 3 wherein said cathode has a low work function coating at the center and a high work function coating at the periphery.

6. An electron discharge device in accordance with claim 4 wherein cathode is provided with a differential heating and heat dissipating means.

7. An electron discharge device in accordance with claim 2 wherein one of said cathode and grid electrodes is of approximately bullet-nose form with the convex portion thereof directed toward the other of said electrodes and wherein both said electrodes are of substantial transverse extent.

8. An" electron discharge device including" a wave guide affording a path for an electron beam lengthwise thereof, a collector electrode opposite one end there of for receiving the electron beam and an electron gun at the opposite end of said wave-guide having an aperture larger than that of the wave-guide and adapted to form an electron beam of substantially uniform electron velocity filling the cross'section area of said wave-guide.

9. An electron discharge device having a cylindrical metal tube constituting a wave-guide affording an electron discharge path, a collector electrode opposite one end of said wave-guide, an electron gun opposite the other end of said wave-guide, and signal input and output coupling devices at the extremities of said wave-guide, said electron gun having apertured electrodes, the apertured electrode closest said other end of said wave-guide having an aperture greater than the cross-section of said wave-guide and said gun having means providing nonlinearly varying electron density transversely of the beam.

10. An electron discharge device including a circular cylindrical wave-guide and means for projecting an electron beam of uniform electron /elocity throughout its cross-section and lengthwise of said wave-guide, said means including electrodes having apertures large enough to form an electron beam whose radius is substantially equal to that of the wave-guide, said means additionally including a grid and cathode assembly comprising means for producing a maximum electron concentration at the center of the beam and a non-linearly diminishing electron density outward along a radius of the beam.

ll. An electron discharge device in accordance with claim 10 in which both said grid and said cathode are substantially flat in parallel planes perpendicular to the path of the electron beam and in which said cathode has a center portion of high emissivity and outer portions of lower emissivity.

12. An electron discharge device in accordance with claim 10 in which said cathode has a center portion of high emissivity and outer portions of lower emissivity.

13. An electron discharge device in accordance with claim 10 in which said cathode has a center portion of low work function and a surrounding portion of high work function.

14. An electron discharge device in accordance with claim 10 in which said cathode incorporates heating and heat dissipating portions producing highest cathode temperature at the center of the emitting surface thereof and reduced cathode temperature at the periphery of the emitting surface thereof.

15. An electron discharge device in accordance with claim 10 wherein said grid and said cathode are of such shape as to be closest at the center of the electron beam and furthest apart at the outer limits of the electron beam.

16. An electron discharge device in accordance with claim 15 wherein said cathode is flat and said grid is a curved figure of revolution convex toward said cathode.

17. An electron discharge device in accordance with claim 15 in which said grid is flat and said cathode is a curved figure of revolution convex toward said grid.

18. An electron beam type amplifying device including an electron gun arranged to produce an electron beam of uniform electron velocity throughout its cross-section, means providing an elongated drift space for said electron beam, a collector at the end of said drift space remote from said electron gun, said electron gun comprising means for producing an electron beam having a high electron density along the axis of said electron beam and having a non-linearly diminishing electron density at points radially outwardly of said axis, and means for coupling signals to said electron beam for amplification by interaction with electron beam.

19. A system for amplifying electromagnetic signals including means for producing a broad electron beam of substantially constant electron velocity throughout its crosssection and of non 'linearly varying space charge density transversely thereof, means for modulating said electron beam near its source with an electromagnetic signal, and means for conducting said signal along said electron beam and providing a drift and interaction space, whereby said signal will be amplified by interaction with said electron beam.

20. A system for amplifying electromagnetic signals including means for producing a broad electron beam of substantially constant electron velocity throughout its cross section, means providing electron-emission diiferentials in said electron beam diminishing from a maximum along. its axis to its outer" extremities so as to cause said electron beam to have a non-linearly varying space charge density transversely thereof, means for modulating said electron beamnear its source with an electromagnetic signal, the means for conducting said electromagnetic signal along said electron beam for interaction therewith.

21. An electron beam type amplifying device including an electron gun arranged to produce a broad electron beam extending along a prescribed beam path and of uniform electron velocity throughout its cross-section, a Waveguide along said beam path providing an elongated drift space for said electron beam, said electron gun providing an electron beam of a section to fill the cross section of said Wave-guide, acollector at the end of said drift space remote from said electron. gun, said electron gun includinga cathode and a further electrode cooperating therewith to iro'duce' in said electron beam at high electron density along said beam path and non-linearly diminishing electron densities at points radially outwardly of said beam path, and means for coupling signals to said electron beam for amplification by interaction with said electron beam in said drift space.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Article by A. V. Haeff, Free. I. R. E., January 1949, pp; 4-10.

Article by Pierce and Field, pages 108-111, inclusive, Proc. I. R. E. for February 1947. 

